I've checked the required wattage from a few different PSU calculators across the internet, and they all averaged around 1000 watts. I'm planning to use this computer for about 5 (or more) years, and I'm not going to O.C. these components for at least the first 2 years I have this. Therefore I'm thinking of getting a 1200W PSU to make up for the 5+ years of component aging. Will it work, or is the 1200 watts not needed for the long-run?

What have you been putting into the PSU calculators, specifically in the capacitor aging field.

A good 850W unit would be more than enough for that system even with overclocking so im not sure why your PSU calculators are telling you 1kW but thats overkill. Capacitor aging unfairly inflates the eXtreme PSU calculators results, most units will never suffer more than 20% capacitor aging over 10 years, let alone their 10% for every year BS, and the newegg one inflates its results significantly.

If you wanted to do three high end cards, or two dual GPU cards, that would be a 1kW-1.2kW setup.

For the most part its just wasting money on something that you wont make full use of and likely never will make full use of, a good 850W unit is more than enough for 99.99% of systems out there, unless you are dropping 3k on a rig with multiple high end GPUs so the $300 PSU is only a small part of the budget its really just spending way too much for no reason. PSUs are also more efficient near about 50-75% load, so running a 1kW unit at 10% load when you are at idle will leave you sucking more power from the wall at idle than a similarly rated 850W unit would as it would be much closer to 20% load.

Yeah, dual GPU cards are the 4870x2, 5970, 6990, GTX 295, and GTX 590, there are some older ones but the 6990 and GTX 590s are the most power hungry cards ever released, they pull more than 300W at full load at stock settings.